I'm working on an online tool for students in my faculty to calculate their final grade for their degrees, and for a while I've had little problems. Students seem to like using the new UI and I've suffered no criticism for a while. Now, I need to add some functionality to the program, meaning the UI will have to chance slightly, but in some very noticeable places.
After watching the whole New Facebook thing kick off, along with the slating of applications with bad UI here on SO and the Ribbon on MS Office receiving mixed reviews I've wondered to myself whether a UI should ever really be changed if users seem to like it. Should a UI be updated after a number of years to keep users interested/be more efficient? Should features be held back in sake of a clean UI?